1. Field of the Invention
This invention relates to ring laser gyroscopes, and more particularly ring laser gyroscopes having self-contained, externally assembled, transverse insertable aperture and Faraday rotator assemblies for use in conjunction with multi-oscillator ring laser gyroscopes.
2. Description of the Related Art
Since its introduction in the early 1960's as a laboratory experiment, the ring laser gyroscope has been developed and used as a commercial product as a logical replacement for the mechanical gyroscope, for use in all manner of inertial guidance systems. Heretofore, the basic two mode ring laser gyroscope has been developed which has two independent electro-magnetic wave modes oscillating in an optical ring cavity. When the ring is stationary, no rotation is ideally indicated. As the ring cavity is rotated about its central axis, the counter rotating waves interact with one another so that a beat frequency is developed. A linear relationship between the beat frequency and the rotation rate of the optical path with respect to the inertial frame of reference may be established. Ideally, the rotation rate is proportional to the beat mode. In this manner, a gyroscope is theoretically produced having no moving parts.
In practice, however, the two mode laser gyroscope often must be mechanically dithered to keep counter rotating travelling waves from locking at low rotation rates. For more information on planar gyroscope two mode lock-in, please see Laser Applications, by Monte Ross, Ed., pp. 133-200 (1971). In an effort to solve this lock-in problem, non-planar ring cavities have been designed containing more than one pair of counter-rotating modes.
These multi-oscillator ring laser gyroscopes have been developed to achieve the goal of an accurate all optical gyroscope having no moving parts. In the multioscillator ring laser gyroscope, the Faraday rotator is a miniature optical element which, when in the presence of a magnetic field, induces a non-reciprocal frequency splitting of right and left-handed circularly polarized beams. This "Faraday splitting" prevents the ring laser gyroscope lock-in phenomenon which is detrimental to the ring laser gyroscope mechanism. Since the Faraday rotator is an intracavity element, it is essential that loss and scatter (due to the rotator) is minimal in order not to limit gyroscope performance. Recently Faraday rotator fabrication technology has developed to a level where production of very low loss, low scatter Faraday rotators is possible. However, in the past, considerable surface degradation and/or contamination incurred at rotator-to-cavity alignment has prevented optimal multi-oscillator ring laser gyroscope performance. For a full discussion of the multi-oscillator ring laser gyroscope, please see Laser Handbook (Vol. IV) Ed. by M. L. Stitch (1985) pp. 229-332.
A non-planar configuration comprising four mirrors and a Faraday rotator is described in Smith, U.S. Pat. No. 4,548,501 issued Oct. 22, 1985.
Critical to the operation of the Faraday rotator is the need to provide a magnetic field propagating through the Faraday element. Heretofore, this critical function has been accomplished by the use of a musket-loaded assembly shown at 42 of FIG. 1. A detailed cross-sectional view of the prior art musket-loaded assembly is shown in FIG. 2. Past rotator alignment procedures have involved the hand-pressing of individual glass and metal components onto a small shelf in the cavity bore. The pressing is performed to seal the various components with indium. Due to the length of time required to align the components to the cavity bore, the number of times the alignment technician must insert the pressing tool, and the inconsistencies involved in hand operation of the pressing tool, this procedure rarely maintains the original condition of the Faraday rotator element. Under these conditions, cavity and rotator contamination is imminent. Furthermore, if the rotator is improperly aligned after being indium-sealed, it must be destructively removed. Generally, if this occurs, the integrity of the assembly components (carrier piece, magnets, etc) is compromised and the entire assembly must be removed and the frame re-cleaned. At this point, there is a high risk that the cavity becomes contaminated.
With reference to FIGS. 1 and 2, heretofore, a musket-loaded assembly 42 was placed into the optical path 48 through the counter-sink mirror mount bore 38. Prior to securing mirror 22 to the counter-sink bore 38 and the gyroscope block frame 12, musket-loaded rotator assembly 42 was placed into the optical path 48 through a telescopic insertion as shown in FIG. 1. The entire assembly 42 is shown in detail in FIG. 2. Prior Art FIG. 2 shows that the rotator assembly 42 is positioned along the optical pathway within the frame 12 and is comprised of a carrier piece 60 which supports a rotator glass piece 50 and defines an aperture 64 for transverse mode suppression of incoming light. The carrier piece 60 and glass piece 50 are surrounded by a plurality of permanent magnets 56, 62, and 58. The annular magnets are stacked on one another and enclosed by ferrous metal rings 52 and 54. Preferably, these ferrous metal rings are made from a non-magnetic ferrous material. The inner cylindrical magnet 56 and the outer cylindrical magnet 58 are each secured to these rings, at 52 and 54, respectively by use of an indium seal. The central cylindrical magnet 62 is substantially thicker than either the inner or outer cylindrical magnets. Each of the magnets are positioned providing an overall super dipole construction. In this manner a relatively uniform magnetic field passes through the Faraday glass piece 50 to perform the Faraday splitting effect.
The musket-loaded assembly illustrated in FIG. 1 and shown in detail in Prior Art FIG. 2, has exhibited considerable surface degradation and contamination which has occurred during the rotator to cavity bore alignment procedure preventing optimal multi-oscillator ring laser gyroscope performance. As may easily be seen with reference to FIG. 1, musket-loading of the rotator assembly 42 into the optical pathway 48C requires that the bore defining the optical pathway 48C be carefully machined to accomodate the insertion of the musket-loaded rotator assembly 42. Furthermore alignment of this assembly must be accomplished at a distance. Musket-loaded assembly is not easily maneuvered within the bore defining the optical pathway portion 48C once the assembly is loaded within the bore. Heretofore, the use of musket loaded rotator assemblies results in a less than optimum Faraday rotator performance during operation.
Attempts to overcome the contamination and alignment problems, as well as the inherent stress placed on the glass piece of the Faraday rotator have been tried but with minimal success. For example, a Faraday bias element together with solenoid magnet is shown as used on page 1463 of J. J. Roland, et. al., Periodic Faraday Bias and Lock-In Phenomena in Laser Gyro, APPLIED OPTICS, Vol. 12, No. 7, 1460-1467 (July 1973). This assembly was used in conjunction with an open optical pathway that is completed with the use of Brewster windows for out-of-plane multi-oscillator ring laser gyroscopes. Likewise, a side-loaded bias package containing a Faraday plate, bias coil, and quarter wave plate for use in a planar ring laser gyroscope was disclosed in a now unclassified U.S. Air Force avionics laboratory technical report AFAL-TR-71-339 (November, 1971). While a side-loaded design which would be simpler than the musket-loaded assembly shown in FIGS. 1 and 2, the integrity of the optical pathway is not clearly maintained.
As an alternative solution to the precision machining needed to produce a passageway constriction or aperture such as 46 along optical pathway 48C of FIG. 1, an unpublished photograph and blueprints for a side-loaded aperture mount was disclosed within Litton during April, 1976. This manner of teaching a side-mounted aperture mount to replace a finely machined restriction within the optical path does not teach the manner in which the side mountable aperture shown in the Litton proprietary materials (which will be presented to the examiner in a subsequently filed Information Disclosure Document) does not disclose any manner for securing the side-loadable aperture mount into the gyroscope block frame.
Additionally, side loaded Faraday rotator assemblies have been suggested for loading into a monolithic multi-oscillator ring laser gyroscope. U.S. patent application, Ser. No. 777,775, filed Mar. 15, 1977 (Thomas J. Hutchings, Inventor) disclosed a scheme for such a side loaded assembly, but is not specific in its teachings as to how such an assembly is constructed nor how it should be sealed to the gyroscope block frame. In this patent, the application suggests the use of element 52 for accomplishing multi-mode operation.
A more recent attempt at providing an alternative structure to the musket-loaded assembly 42 of FIG. 1 was discussed and disclosed in U.S. Pat. No. 4,284,329 issued to Smith Aug. 18, 1981. FIGS. 6, 6A, and 6B of the '329 Smith patent disclosed a Faraday rotator 156 which is comprised of the Faraday rotator mount 154 having a central portion with one end flanged to restrain lateral movement of the device within the aperture 120 provided in the laser gyro block 102. The other end of the Faraday rotator mount 154 is cut away to leave a platform for mounting active components such as the permanent magnet 166, the Faraday rotator slab 165, and the pole piece 170 which forms a carrier function similar to the carrier piece shown in this application's FIG. 2. A coil spring 168 is forced against the Faraday slab 165 within the inner diameter of annular permanent magnet rings 166 (or a plurality of rings 172 and 176 positioned in opposing dipole directions along the axis of the optical pathway), surrounding the Faraday rotator 165 and the coil spring 168. The purpose of the coil spring is to hold the rotator slab 165 against the Faraday rotator mount 154. Smith teaches that the Faraday rotator mount 154 is preferably formed of the same material as the laser gyro block 102. Smith also teaches that the Faraday rotator 156 performs a second function. Because of the close fit provided within the aperture 120 in the gyro block 102, the Faraday rotator 156 blocks the longitudinal flow of gas through the passage 112. This, as Smith explains, prevents a net circulation of gas through the closed path which he believes acts to substantially reduce the possibility of circulation of scatter particles carried by the gas. Although Smith discloses a method of assembly of the Faraday rotator into the gyro block 102 body, Smith does not teach how the Faraday rotator mount 152 will be secured to the gyro block 102, nor does Smith address the problems of stress and deformation to the Faraday rotator slab 165 provided by the coil spring 168 to the surface of the Faraday rotator slab 165. Also it is unlikely that the Faraday rotator mount as taught by Smith will substantially reduce contamination being introduced into the optical pathway within the gyro block 102.